A piezoelectric actuator, which expands and contracts by charging and discharging, is used to linearly move a piston or the like. For instance, as disclosed in JP-A-2002-136156, a fuel injection system for an internal combustion engine uses such a piezoelectric actuator so that an on/off-valve of a fuel injector for fuel injection is switched thereby.
More specifically, as illustrated in FIG. 10A, a driving device is constructed to charge and discharge a piezoelectric actuator to expand and contract it. In this driving device, a series circuit 3 of an inductor 1 and a piezoelectric actuator 2 is provided with: a charging path 6 for supplying power from the positive side of a direct current power source 5 through a charging switch 4 constructed with MOSFET; and a discharging path 8, connected in parallel with the series circuit 3, for discharging the charged electric charges of the piezoelectric actuator 2 through a discharging switch 7 constructed with MOSFET.
The piezoelectric actuator 2 in this example is provided in an injector for injecting fuel into any of the multiple cylinders of an internal combustion engine and opens the injector when it is expanded. Other multiple piezoelectric actuators are connected in series with the inductor 1 and in parallel with the piezoelectric actuator 2. For this reason, the terminal (negative terminal) of each of the parallel-connected piezoelectric actuators 2 on the opposite side to the inductor 1 is connected to a ground on the negative side of the direct current power source 5 through a selector switch 9. Each selector switch (also referred to as a cylinder selector switch) 9 is for selecting one of the parallel-connected piezoelectric actuators 2 as an object to be driven. The selector switch 9 is kept ON during a period in which fuel is injected into the cylinder.
In this example, the selector switch 9 is also MOSFET, and the diodes 4a, 7a, 9a are parasitic diodes of the respective MOSFETs forming the switches 4, 7, 9.
When a driving signal instructing driving of the piezoelectric actuator 2 is externally inputted, the selector switch 9 is turned on and further charging switching control is carried out. In this charging switching control, the charging switch 4 is repeatedly turned on and off with the discharging switch 7 kept OFF, so that the piezoelectric actuator 2 is thereby charged and expanded. When the input of the driving signal is thereafter ceased, discharging switching control is carried out. In this discharging switching control, the discharging switch 7 is repeatedly turned on and off with the charging switch 4 kept OFF, so that the piezoelectric actuator 2 is thereby discharged and contracted.
During the charging period, specifically, the following operation is repeated: the charging switch 4 is turned on with the discharging switch 7 OFF and a charging current is thereby led to flow from the direct current power source 5 to the piezoelectric actuator 2 through the charging path 6; thereafter, the charging switch 4 is turned off; and a charging current (i.e., flywheel current) passed by energy accumulated in the inductor 1 is thereby led to flow. This charging current flows from the negative side to the positive side of the piezoelectric actuator 2 through the parasitic diode 7a of the MOSFET forming the discharging switch 7. The piezoelectric actuator 2 is charged stepwise by repeating the above procedure.
During the discharging period, meanwhile, the following operation is repeated: the discharging switch 7 is turned on with the charging switch 4 OFF and a discharging current is thereby led to flow from the positive side of the piezoelectric actuator 2 to the discharging path 8 through the inductor 1; thereafter, the discharging switch 7 is turned off; a discharging current is thereby led to flow from the positive side of the piezoelectric actuator 2 to the direct current power source 5 through the inductor 1 and the parasitic diode 4a of the MOSFET forming the charging switch 4; and the electric charges in the piezoelectric actuator 2 is recovered to the direct current power source 5 by this discharging current. The piezoelectric actuator 2 is discharged stepwise by repeating the above procedure.
In this type of driving device, as illustrated in FIG. 10B, the charging voltage (hereafter, also referred to as a piezoelectric voltage) of the piezoelectric actuator 2 is monitored while the above discharging switching control is being carried out. When it is determined that the charging voltage has become equal to or lower than a discharge termination target value set slightly higher than 0V, the discharging switching control is terminated and the discharging switch 7 is kept OFF.
The discharging current in FIG. 10B refers to the discharging current of the piezoelectric actuator 2. FIG. 10B illustrates the waveform of the discharging current and the state of turn-on and turn-off of the discharging switch 7 observed when the following control is carried out as the discharging switching control: each time the discharging current has risen to a preset upper-limit value after a driving signal is changed to the low level, the discharging switch 7 is caused to switch from ON to OFF; and each time the discharging current has fallen to a preset lower-limit value, the discharging switch 7 is caused to switch from OFF to ON.
When the piezoelectric actuator 2 is discharged by the discharging switching control, the capacitance of a piezoelectric element forming the piezoelectric actuator 2 varies. Therefore, the piezoelectric voltage fluctuates and drops as illustrated in FIG. 11A.
For this reason, variation is produced in the piezoelectric voltage after the termination of discharging switching control. As a result, the piezoelectric voltage drops to a value close to 0V in some cases and does not in other cases.
With this variation in piezoelectric voltage, the following takes place when charging switching control is carried out next to drive the piezoelectric actuator: the time from when the charging switching control is started to when the piezoelectric actuator starts to expand and the time it takes the piezoelectric actuator to expand by a desired amount vary. This degrades the driving accuracy.
As a fuel injection system for a vehicle diesel engine, a diesel common rail system (hereafter, abbreviated as CRS) also uses a piezoelectric actuator in an injector so that high-pressure fuel accumulated in a common rail is injected from the injector.
Because of the fuel pressure in the common rail, pressure is applied to the piezoelectric element forming the piezoelectric actuator 2, and this pressure may produce voltage in the piezoelectric element by the piezoelectric phenomenon. For this reason, after the termination of discharging switching control, the piezoelectric voltage is gradually increased as illustrated at part A in FIG. 11B. The quiescent period (injection interval) from the termination of the current fuel injection to the start of the next fuel injection varies according to the operating state of the engine or the like as illustrated at part B in FIG. 11B. This leads to variation in the piezoelectric voltage when charging switching control is started for the next fuel injection.
Consequently, also in the CRS, the time for the piezoelectric actuator 2 to expand by a desired amount after the start of charging switching control varies. As a result, the injector opening timing varies and this varies the fuel injection quantity.
The CRS is so constructed that multistage injection, in which fuel is injected into one cylinder more than once, is carried out to more finely control the injection quantity and the injection timing to suppress deterioration in emission. When the accuracy of driving of the piezoelectric actuator is degraded, the CRS cannot attain expected performance.